Integrated circuit dice often contain devices (e.g., power MOSFETs) that generate a considerable amount of heat. When the dice are assembled into semiconductor packages, they are normally encased in a plastic molding compound, and this can make it difficult to remove that heat.
In wire-bonded packages, the heat removal can be facilitated by mounting the die onto a heat slug. For example, FIG. 1A is a cross-sectional view of a SOT-like package 1 that contains an integrated circuit die 3 encased in a plastic molding compound 5. SOT, an acronym for “small outline transistor”, is a common plastic leaded package for housing semiconductor devices. Electrical connections between die 3 and leads 2A and 2C are made via bonding wires 4A and 4B, which are also embedded in molding compound 5. Leads 2A and 2B extend from the sides of molding compound 5 and are bent downward to form mounting surfaces that contact the backside surface 6 on which package 1 is mounted (e.g., a printed circuit board). To assist in heat removal, die 3 is mounted on a metal heat slug 2B. To insure good heat conduction from the die into the leadframe, die 3 has no backside oxide often requiring special steps to remove the backside oxide or to thin the wafer through mechanical grinding. Such a bottom surface may be referred to as the die's “bare” backside.
In many package implementations, leads 2A and 2C are not coplanar with the top of heat slug 2B. A bottom surface of heat slug 2B is exposed and also contacts the mounting surface 6. Since heat slug 2B is made of metal and has a relatively large cross-sectional area, it provides a broad, low-resistance thermal path by which heat generated in die 3 can escape to backside surface 6.
Similarly, FIG. 1B shows a cross-sectional view of a dual flat no-lead (DFN) package 11, which a die 13 is mounted on a heat slug 12B. Die 13 is connected to leads 12A and 12B by means for bonding wires 14A and 14B. Unlike leads 2A and 2B in package 1, leads 12A and 12B have external surfaces that are flush with the surfaces of molding compound 15. In particular the bottom surfaces of leads 12A and 12B are coplanar with the bottom surface of molding compound 15, allowing leads 12A and 12B to make direct contact with circuit elements on surface 6. Die 13 is mounted on a metal heat slug 12B, which is similar in structure to heat slug 2B, and provide a broad thermal path for heat to escape from die 13 to mounting surface 16. In many package implementations, leads 12A and 12C are not coplanar with the top of heat slug 12B.
FIG. 3A shows a plan view of package 11 (FIG. 1B is taken at cross-section 1B-1B shown in FIG. 3A). As shown, leads 12A, 12D, 12F and 12H are arranged in a row along a side 17A of molding compound 15 and leads 12C, 12E, 12G and 12I are arranged in a row along an opposite side 17B of molding compound 15. Bonding wires 14A and 14C-14I are also shown. Tie bars 16A and 16B originally connected heat slug 12B to the leadframe of which it was a part before package 11 was singulated.
In packages 1 and 11, relatively thin bonding wires are used to make electrical contact with pads (not shown) on the top surface of dice 3 and 13. These bonding wires can introduce a significant amount of resistance into the connections between the dice and the leads, and they are vulnerable to breakage. A more robust electrical connection with the pads can be made by turning the dice upside down so that the contact pads are facing downward, and making the connections with metal bumps or balls. FIGS. 2A and 2B illustrate cross-sectional views of SOT-like and DFN packages that are similar to packages 1 and 11, except that they are bump-on-leadframe (BOL) or “flip-chip” packages. SOT-like package 21, shown in FIG. 2A, contains a die 23 that is connected to leads 22A and 22B by means of metal bumps 24A and 24B. Die 23 and metal bumps 24A and 24B are encased in molding compound 25, and leads 22A and 22B extend from molding compound 25 in a manner similar to leads 2A and 2B in package 1. DFN package 31, shown in the cross-sectional view of FIG. 2B, contains a die 33 that is connected to leads 32A and 32B by means of metal bumps 34A and 34B. Die 33 and metal bumps 34A and 34B are encased in molding compound 35, and leads 32A and 32B have external surfaces that are flush with the surfaces of molding compound 35 in a manner similar to leads 12A and 12B in package 11.
FIG. 3B shows a plan view of package 31 (FIG. 2B is taken at cross-section 2B-2B shown in FIG. 3B). As shown, leads 32A, 32C, 32E and 32G are arranged in a row along a side 37A of molding compound 35 and leads 32B, 32D, 32F and 32H are arranged in a row along an opposite side 37B of molding compound 35. Metal bumps 34A-34H are also shown.
In the bump-on-leadframe packages 21 and 31, it is not feasible to provide a thermal escape path by mounting the dice 23 and 33 onto a heat slug in the manner of dice 3 and 13 in wire bond packages 1 and 11 because the back of the die does not face down toward the bottom of the package. Instead the die is “suspended”, i.e. supported by bumps 24 or 34 acting as pillars and has its backside facing “up”, away from the bottom of the package. Even if a heat slug were included in the package, there would be no obvious means to connect the bumps to the heat slug since the bumps or pillars are located at the die's periphery and the heat slug is located near the center of the die, and because in many exposed pad packages, the top of the heat slug is not coplanar with the leads.
What is needed, therefore, is a technique for combining the electrical advantages of a BOL package with the thermal advantages of mounting the die onto a heat slug.